1. Field of the Invention
The invention relates to a cordless telecommunication apparatus conversion means for converting an analog RF signal into a digital data stream and having synchronization means for synchronizing the data stream with respect to an apparatus clock, characterized in that the conversion means comprise comparator means which compare the RF signal with a reference signal generated from the former by means of integration and generate the data stream in accordance with the result of the comparison, and in that detection means are provided which, in the case of the first succession of two identical bit values of the synchronized data stream, control the comparator means in such a manner that the integration of the RF signal is interrupted and the integration of the RF signal sections corresponding to the two successive bit values is compensated for.
2. Description of the Related Art
The technical development of communication systems that include cordless telecommunication apparatuses for the cordless transmission of voice and non-voice information is tied to various standards, analogously to the ISDN standard (Integrated Services Digital Network) which has existed for some time in line-connected communications. Apart from some national standards and several cross-boundary standards such as the CT1, CT1+ standard on an analog basis and the CT2, CT3 standard on a digital basis, a standard, the so-called DECT standard (Digital European Cordless Telecommunication; compare European Telecommunication Standard--Final Draft--, prETS 300 175-1 . . . 9, 5/1992, ETS Institute 06921 Sophia Antipolis, France and Philips Telecommunication Review, Vol. 49, No. 3, 9/1991, pages 68 to 73, "DECT, a universal cordless access system" by R. J. Mulder), analogous to the global GSM standard (Group Speciale Mobile or Globals Systems for Mobile Communication; compare Informatik Spektrum Vol. 14, No. 3, 6/1991, pages 137 to 152, "Der GSM--Standard--Grundlade fur digitale europaische Mobilfunknetze") (The GSM Standard--Basis for Digital European Mobile Radio Networks) by A. Mann) for mobile radio, has been created on a European scale for the lower-power cordless communication between portables and a base station, with ranges of some 100s of meters. It is an essential feature of the DECT standard that the base station can be connected to line-connected communication networks (e.g. PSTN=Public Switched Telephone Network; PTN=Private Telecommunication Network).
For cordless communication according to the DECT standard, a dynamic channel selection of approximately 120 available channels is carried out. The 120 channels result from the fact that in the DECT standard, ten frequency bands between 1.8 and 1.9 GHz are used, a time-division multiplex frame of 10 ms being used in time-division multiplex access (TDMA) in each frequency band according to the representation in FIG. 1. In this time-division multiplex frame, 24 time channels (from 0 to 23) are defined, which provide a frame structure. This frame structure is then used in such a manner that for each frequency band, 12 mobile stations MS with a base station BS of a DECT communication system can operate simultaneously in duplex mode (MS-BS and BS-MS or, respectively, BS-MS and MS-BS). A time slot of in each case 417 .mu.s is allocated to the 24 time channels.
This time slot specifies the time in which information (data) are transmitted. This type of transmitting information in duplex mode is also called the ping-pong method because transmission takes place at a particular time and reception takes place at another time. In this ping-pong method, one time frame or pulse (burst) of 365 .mu.s is transmitted in each time slot, which approximately corresponds to a frame length of 420 bits, with a data throughput of 42 kbit/s. Taking into consideration that in each case 30 bits are available in a guard space at both ends of the time frame in order to avoid overlaps by adjoining time slots, this results in a total data throughput of 1,152 MBit/s referred to the time-division multiplex frame.
According to FIG. 2, the succession in time of the pulses transmitted per time-division multiplex frame defines a PH channel, the so-called PHysical channel which is allocated to a so-called PHysical layer (PH-L). The data packet of 420 bits transmitted in this channel is called the PH packet and allocated to a D field. Of the 420 data bits (sequence of H/L bit values) in the PH packet, 32 bits are used for synchronization and 388 bits are used for transmitting net information (NI). The 32 bits for the synchronization are subdivided, in turn, into two data bit sequences of in each case 16 bits. The first data bit sequence (sequence with the first 16 H/L bit values) is a synchronization initiation word SY-EW by means of which the synchronization is initiated. In the ideal case, this synchronization initiation word SY-EW consists of a periodic "101" or "HLH" sequence for a "mobile station MS-base station BS" direction of transmission" and of an also periodic "010" or "LHL" sequence for the reverse "base station BS-mobile station MS" direction of transmission. The bracketed allocations are alternative allocations depending on what sequence is allocated to which direction of transmission.
The second data bit sequence (sequence with the second 16 H/L bit values) is a synchronization confirmation word SY-BW, with which the synchronization initiated with the synchronization initiation word SY-EW must be confirmed. In this confirmation, the synchronization confirmation word SY-BW must be almost, i.e. essentially every data bit, recognized. The synchronization initiated with the synchronization initiation word SY-EW is only accepted when this is so. In this arrangement, synchronization is initiated when it can be assumed with a certain probability that the synchronization initiation word SY-EW is a "HLH" or "LHL" sequence.
In addition, yet more layers are defined in the DECT standard, analogously to the ISDN standard with the ISO/OSI 7-layer model. One of these layers is a Medium Access Control Layer (MAC-L) which was allocated the 388 bits for the transmission of the net information in an A field and in a B field according to FIG. 3. The A field here comprises 64 bits which, inter alia, are used for messages when joining the base and mobile stations of the DECT communication system. The remaining 324 bits of the B field, 320 bits of which are used for voice data and 4 bits for detecting partial interferences of the pulse, are allocated to other ISO/OSI layers.
In its simplest form, the DECT communication system has a base station with at least one mobile station. More complex (e.g. networked) systems contain several base stations having in each case several mobile stations. Due to the 24 time channels defined in the DECT standard, up to 12 mobile stations can be allocated to the base station, which communicate with the base station in duplex mode. For the time-division multiplex frame of 10 ms, also defined in the DECT standard, duplex mode means that information is transmitted every 5 ms from the base station to a mobile station or conversely.
FIG. 4 shows a cordless communication arrangement KA which is typical of DECT communication systems and in which the mobile station MS is used as a transmitter SG and the base station BS is used as a receiver EG. On the basis of the above information, the cordless communication arrangement KA can also be modified to the extent that the base station BS is used as the transmitter SG and the mobile station MS is used as the receiver EG. The transmitter SG has a transmitting antenna SA via which a radio signal FS generated by the transmitter SG is sent to the receiver EG. To be able to receive the radio signal FS, the receiver EG has a receiving antenna EA.
According to the DECT transmission convention forming the basis of the DECT communication system, the radio signal FS is a radio-frequency carrier signal with a carrier frequency between 1.8 and 1.9 GHz which is modulated with a digital transmitted data stream SDS present in the transmitter SG with a transmitter-specific clock (phase). The digital data stream present in the transmitter SG contains the information required for the cordless transmission. This information includes, inter alia, the abovementioned synchronization and net information which, according to the DECT standard, is contained, for example, in an information packet (PH packet) of 420 bits. Using this information or data packet, the modulated radio signal FS (carrier signal) is then generated in the transmitter SG and transmitted at regular time intervals, predetermined by the time-division multiplex frame ZMR, for a period predetermined by the time slot in accordance with the DECT transmission convention.
To be able to decode the transmitted information packet (transmitted voice information per time slot TS), the radio signal FS (modulated carrier signal) must be demodulated in the receiver EG. After demodulation, a digital received data stream EDS is produced in the receiver EG, which data stream, in the event of an error-free transmission of the radio signal FS, has the same bit structure as the transmitted data stream SDS (right-hand received data stream EDS) and which has a different bit structure (left-hand received data stream) from the transmitted data stream in the case of a transmission of the radio signal with errors.
For this demodulation, an analog/digital converter is used in a familiar manner, in accordance with the GSM standard in the case of mobile transceivers. This analog/digital converter samples an analog signal bit by bit with an adjustable sampling rate. However, since these analog/digital converters are quite expensive, there is an interest in less inexpensive solutions for the demodulation of modulated analog signals in a cordless telecommunication apparatus in many areas of cordless telecommunication technology (for example in the transmission of voice signals). From the British Patent reference GB-2 238 922 A, a cordless telephone is known which exhibits conversion means for converting an analog RS signal into a digital data stream and synchronization means for synchronizing the data stream with respect to an apparatus clock.
From the European Patent reference EP-0 124 166 A2, a wave shape converter circuit is known which generates from a frequency-modulated sinusoidal analog signal with varying "peak-peak" amplitude values and direct-voltage potential values a square wave signal having a uniform (constant) pulse duty ratio. For this purpose, the converter circuit includes an analog/digital comparator, at the inverting input of which the analog signal is present and at the noninverting input of which a reference signal generated from the analog signal by integration is present. The square wave signal present at the output of the analog/digital comparator is fed back via a D-type flip flop and filters following it at the output back to the inputs of the analog/digital comparator in such a manner that the pulse duty ratio of the square wave signal is constant with an analog signal which changes with respect to the direct-voltage potential values.
From the European Patent reference EP-0 133 067 A1, a device for regenerating a readout signal from optical storage diskettes is known which exhibits a deformation circuit following a readout processor for correcting a readout clock signal. The deformation circuit is constructed as a phase monitoring loop for which a switched integrator circuit and a comparator are used.